Neurological disorders including Huntington's disease, Parkinson's disease and Alzheimer's disease represent a major unmet medical need. In some cases, these diseases are monogenic, making them ideal targets for oligonucleotide therapeutic intervention, e.g., RNA interference (RNAi). RNAi is a fundamental mechanism involving short double stranded RNA fragments that can be used to reprogram cellular machinery and silence and degrade targeted mRNA on demand. This technology is clinically advanced and has revolutionized the field of human functional genetics.
Many different technologies have been explored for mRNA knockdown both as therapeutics and as tools for functional study, including viral based delivery of short hairpin RNAs (shRNAs), antisense oligonucleotides (ASOs), and naked or slightly modified siRNAs (Sah, D. W. Y. & Aronin, N. Oligonucleotide therapeutic approaches for Huntington disease. J. Clin. Invest. 121, 500-507 (2011); DiFiglia, M. et al. Therapeutic silencing of mutant huntingtin with siRNA attenuates striatal and cortical neuropathology and behavioral deficits. Proceedings of the National Academy of Sciences of the United States of America 104, 17204-17209 (2007)).
ASOs have also shown to be a promising approach. This technology exhibits efficient delivery to cells without a delivery vehicle and has been administered to brain for the treatment of Huntington's disease for successful knockdown in both rodent and non-human primate brains (Mantha, N., Das, S. K. & Das, N. G. RNAi-based therapies for Huntington's disease: delivery challenges and opportunities. Therapeutic delivery 3, 1061-1076 (2012); Kordasiewicz, H. B. et al. Sustained Therapeutic Reversal of Huntington's Disease by Transient Repression of Huntingtin Synthesis. NEURON 74, 1031-1044 (2012)). Unfortunately, current studies show that a 700 μg cumulative dose administrated over two weeks is required to see just 50% silencing (Kordasiewicz, Supra).
Unmodified siRNA (“naked siRNA”) has been difficult to deliver to more sensitive cell lines and in vivo to tissue in the past. Although transfection reagents such as Lipofectamine can be used, there is a very narrow window within which it is efficacious and non-toxic, and it must be optimized independently for different batches of neurons to determine siRNA to lipid ratios necessary for comparable levels of silencing (Bell, H., Kimber, W. L., Li, M. & Whittle, I. R. Liposomal transfection efficiency and toxicity on glioma cell lines: in vitro and in vivo studies. NeuroReport 9, 793-798 (1998); Dass, C. R. Cytotoxicity issues pertinent to lipoplex-mediated gene therapy in-vivo. Journal of Pharmacy and Pharmacology 1-9 (2010); Masotti, A. et al. Comparison of different commercially available cationic liposome-DNA lipoplexes: Parameters influencing toxicity and transfection efficiency. Colloids and Surfaces B: Biointerfaces 68, 136-144 (2009); Zou, L. L. et al. Liposome-mediated NGF gene transfection following neuronal injury: potential therapeutic applications. Gene Ther 6, 994-1005 (1999)). Hydrophobically modified siRNAs have also been used as an alternative for cellular and brain delivery (Sah, Supra; Soutschek, J. et al. Therapeutic silencing of an endogenous gene by systemic administration of modified siRNAs. Nature 432, 173-178 (2004); Cheng, K., Ye, Z., Guntaka, R. V. & Mahato, R. I. Enhanced hepatic uptake and bioactivity of type alpha1(I) collagen gene promoter-specific triplex-forming oligonucleotides after conjugation with cholesterol. Journal of Pharmacology and Experimental Therapeutics 317, 797-805 (2006); Byrne, M. et al. Novel Hydrophobically Modified Asymmetric RNAi Compounds (sd-rxRNA) Demonstrate Robust Efficacy in the Eye. Journal of Ocular Pharmacology and Therapeutics 29, 855-864 (2013)), and some of these compounds have even made it to clinic, but ensuring both chemical stability and minimal toxicity while maximizing delivery remains a difficult task. Current hurdles in RNAi technology limit its ability to be used for both functional genomics studies and therapeutics, providing an opportunity for improvement to their design as it applies to the area of neuroscience both in vitro and in vivo.